1. Field of the Invention
The present invention relates to a variable gain amplifier capable of adjusting a gain according to a level of an input signal, to a solid-state imaging device for converting an optical signal into a digital signal and outputting the digital signal, which is used for a video camera, an electronic camera, an image input camera, a scanner, a facsimile or the like, and to an optical signal reading method.
2. Description of the Related Art
A semiconductor image sensors such as a CCD image sensor and a MOS image sensor are excellent in mass production, and thus such sensors have been applied to many image input devices along with an advance in a fine patterning technology.
Especially, in recent years, a CMOS image sensor has been a main focus of attention because of its advantages in contrast with the CCD image sensor, i.e., smaller power consumption, and capability of fabricating a sensor device and a peripheral circuit device by the same CMOS technology.
Such a CMOS image sensor is described in U.S. Pat. No. 6,128,039. The CMOS image sensor disclosed in U.S. Pat. No. 6,128,039 is called an active pixel sensor. As shown in FIG. 1 transferred from this US patent, in combination with an active load such as a constant current source M4 or the like, a signal voltage is outputted by a source follower.
In the CMOS image sensor of U.S. Pat. No. 6,128,039, a load capacitor C1 for storing signal charges and a gate-source capacitor Cgs of the reading transistor M2 are connected in series to a gate of M2. These capacitors are set parallel in a fixed capacitor for charge/voltage conversion, and the fixed capacitor for charge/voltage conversion is seemingly changed in the capacitance. Charging of the capacitor C1 connected to the source of M2 greatly changes a source potential during signal reading. As this potential change is returned through the capacitor Cgs to the gate, an input potential is also changed, causing a considerable deterioration of linearity of a transmission characteristic. Consequently, in this CMOS image sensor, the constant current source M4 as a load of a reading transistor M2 was inevitable.
In addition, in recent years, a column type analog/digital converter of an integration system (hereinafter referred to as a column type ADC) has been mounted on the image sensor. In such a column type ADC, an analog electric signal (photoelectric signal) into which an optical signal is converted is compared with a comparison lamp voltage having a predetermined gradient by a comparator, and is converted into a pulse count corresponding to an amplitude of the photoelectric signal by a pulse counter.
In such a case, when the analog signal is small, a gradient of the comparison lamp voltage is also reduced to secure a dynamic range.
Furthermore, in a solid-state imaging device including a number of unit pixels arrayed in horizontal and vertical directions, sampling pixels are thinned to output only detecting signals of remaining pixels not thinned during moving image reproduction, thereby increasing a frame rate.
However, in the case of the image sensor including the constant current source M4 for each column, since there are variances in characteristics among the respective constant current sources M4, variances are generated in gains among columns. This variance appears as an offset difference between columns, and when seen on a screen, the variance appears as a so-called vertical fixed pattern noise.
When a signal voltage outputted from the conventional image sensor is inputted to the column type ADC, a gradient of a comparison lamp voltage is reduced corresponding to a small analog signal. In this case, a limitation is placed on an S/N ratio because of linearity of a lamp signal or comparison accuracy of the comparator, and because of an influence of an offset voltage generated in a photoelectric conversion device or the like.
Furthermore, if sampling pixels are thinned in the conventional image sensor, a modulation transfer function (MTF: resolution) is deteriorated, forming an image of much moire. In addition, when a reduction is made twice or lower as large as a sampling frequency proportional to an inverse number of a sampling pixel interval because of the thinning of the sampling pixels, a folding noise (aliasing noise) may be thus generated. Also, as it is necessary to operate the image sensor at a high speed according to the number of pixels, power consumption is increased.
This present invention is performed considering the foregoing drawbacks.
An object of the present invention is to provide a variable gain amplifier, a solid-state imaging device, and an optical signal reading method being capable of improving an S/N ratio while enhancing a dynamic range when a photoelectric signal is digitized.
Another object thereof is to provide a solid-state imaging device and an optical signal reading method being capable of reducing fixed pattern noises and further suppressing a reduction in resolution and generation of folding noises while maintaining a low power consumption operation by thinning sampling pixels.
A variable gain amplifier of the present invention is characterized in that it converts a first signal voltage and a second signal voltage into charges by sequentially inputting the first and second signal voltages, generates a difference signal between the first and second signal voltages, amplifies the difference signal by a gain so as to set the difference signal within a requested range of a digital encoding analog input level, and outputs the difference signal.
A variable gain amplifier comprises a so-called chopper type switched capacitor integrating circuit. The chopper type switched capacitor integrating circuit includes, as shown in FIG. 2 as the example,: an operational amplifier 31 having a positive input terminal (+) to which a reference voltage Vref is applied and a negative input terminal (xe2x88x92), and an output terminal; an input capacitor Ci (C1) provided in a signal path extending from the input terminal of the variable gain amplifier 105a to the negative input terminal (xe2x88x92) of the operational amplifier 31; a feedback capacitor Cf composed of a plurality of capacitors (C2, C3, C4, . . . etc.) provided between the negative input terminal (xe2x88x92) and output terminal of the operational amplifier 31; first switch devices SW1 and SW2 for connecting/disconnecting a signal path extending from the input terminal of the variable gain amplifier 105a to the other end of the input capacitor Ci; a second switch device SW3 for turning ON/OFF (connecting/disconnecting) an input of the reference voltage Vref to the other end of the input capacitor Ci; and a third switch device SW4 for connecting/ disconnecting a signal path between the negative input terminal (xe2x88x92) and output terminal of the operational amplifier 31.
An amplifying gain of the operational amplifier 31 can be adjusted as follows.
That is, the capacitors C2, C3, C4 . . . etc. constituting the feedback capacitor Cf are connected to switch devices (SW5, SW6 . . . etc.) for controlling connection/disconnection of the respective capacitors between the input and output terminals of the operational amplifier 31. Accordingly, by selectively connecting/disconnecting the switch devices (SW5, SW6 . . . etc.), a proper capacitor is selected and connected between the input and output terminals of the operational amplifier 31. Therefore, by increasing/reducing a capacitance of the feedback capacitor Cf, a ratio (Ci/Cf) of the input capacitor Ci with respect to the feedback capacitor Cf is adjusted. With this, it is possible to adjust the gain of the operational amplifier 31.
A solid-state imaging device comprises photoelectric conversion devices arrayed in rows and columns, the foregoing variable gain amplifier provided for each of the columns and connected to outputs of the photoelectric conversion devices of each column, and an analog/digital conversion circuit for converting a difference signal into a digital signal, which is connected to an output side of the variable gain amplifier. In this case, a first signal voltage to be inputted to the variable gain amplifier is obtained by converting an optical signal into an electric signal, and a second signal voltage is obtained when the photoelectric conversion device is initialized.
The optical signal reading method is characterized in the followings. That is, the first signal voltage is converted into charges and then charges are stored, followed by outputting the second signal voltage and then converting it into charges. And the difference signal between the first signal voltage stored as charges and the second signal voltage converted into charges is amplified with the gain such that the difference signal is within the requested range of the digital encoding analog input level.
In other words, even when amplitude of the analog difference signal is smaller than the proper range of the digital encoding analog input level, the analog difference signal can be amplified to be properly set within the range of the digital encoding analog input level. As a result, when the analog difference signal is digitized, it is possible to secure the dynamic range and to improve the S/N ratio.
Moreover, the foregoing solid-state imaging device includes the heavily doped buried layer for storing photo-generated charges (holes), which is provided below the channel of the MOS transistor for signal detection so as to surround the source region. And the foregoing solid-state imaging device is characterized by its capability of reading out the optical signal even without connecting any active loads such as a constant current source to the source which is the output end of the pixel 101. In this event, since a gate potential is maintained constant by an external power source, the surface potential is uniquely decided by the photo-generated holes stored in the heavily doped buried layer. As a state of the photo-generated holes stored in the heavily doped buried layer is not influenced by a source potential, no unnecessary feedback operations are applied on the heavily doped buried layer even if the source potential is changed during the signal reading. Therefore, the surface potential can be accurately transmitted to the source even if the load of the source follower is only a capacitor capable of being readily matched to each other for characteristics, not a constant current source in which it is difficult to match to each other for characteristics. With this, it is possible to read out a signal while suppressing a fixed pattern noise.
According to another present invention, the difference signal generation circuit is provided, as shown in FIG. 7 as an example, with a pixel mixing switch device SM which makes connection/disconnection of the amplifiers 105a, 105b of at least two columns, more in detail of the negative input terminals of the operational amplifiers 31 thereof. This structure is adequate to the solid-state imaging device which treats moving image.
Further, the thinning operation can be performed with the above structure by connecting the pixel mixing switch device SM to mix signals from pixels of at least two columns and average them.
Thus, even if thinning is carried out during scanning, since an averaged signal is outputted as a signal in the position of the pixel 101 thinned during scanning, deterioration of MTF can be prevented. Further, since no reduction occurs in a sampling frequency, it is possible to prevent generation of the folding noises and obtain a fine image. Moreover, since the number of times of signal processing is reduced according to the thinning, it is possible to prevent the power consumption from increasing.